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Variation
Name:
2.5 Genetics
2.5.7 Causes of variation
Learning Objectives
1. Define variation and mutation
2. List the causes of variations and mutations
3. List the types of mutations
4. Discuss the causes for and effects of increased mutation rates
Taxonomy – is the science of classifying organisms.
Classification is the placing of organisms into groups, based on similar characteristics.
This simplifies the study of organisms and allow scientists to communicate with each other.
The basic unit of classification is the species.
A species is a group of organisms that are capable of interbreeding to produce fertile offspring.
(Liger = sterile offspring of a male lion and a female tiger)
(A mule is the sterile offspring of a male donkey and a female horse)
VARIATION
Variation means the differences in characteristics between members of the same species.
Variations can be acquired (‘picked up’ during the life of the organism) e.g. riding a bicycle or inherited
(genetic – can be passed on to next generation) e.g. tongue rolling.
Causes of inherited variation:
 Sexual reproduction
Gamete formation - independent assortment of homologous chromosomes during meiosis ensures genetic
variation among gametes.
Crossing over - of linked genes, during meiosis, enhances genetic variation
Recombination - random fertilisation between gametes from different parents ensures variation.

Mutation - this is any sudden change in the amount or structure of DNA.
CAUSES OF MUTATIONS
Spontaneous mutations- Faulty DNA replication making ‘mistakes’ or when DNA fails to repair properly.
Mutagens - agents that cause mutations (speed up spontaneous rate of mutation e.g.
 Ionising radiation such as X-rays, UV rays, cosmic rays, //gamma rays. Harm DNA indirectly
and effect can accumulate in body over years. Can also harm gametes. UV alters DNA structure
directly & cause mutations in skin cells – skin cancer.

chemicals e.g. formaldehyde, tobacco smoke, caffeine and many drugs, preservatives and pesticides.
Many are carcinogenic.

Some viruses and ageing may cause mutations. Hepatitis B virus can cause liver cancer.
Somatic cell mutations may give rise to benign tumours e.g. warts or malignant tumours (cancer) but the
mutation is not inherited. Sex cell mutations are inherited.
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Variation
There are 2 types of mutation:
Gene mutation:
a change in bases in the gene. This alters the amino acid sequence of the protein controlled by that gene.
The bases may be altered by deletions, insertions and substitutions. They usually make the gene nonfunctional or recessive.
e.g.
 Sickle cell anaemia – an inherited blood disorder caused by a mutation in the haemoglobin
gene. Mutation is common in black people in tropical climates.
If homozygous for disease (HbSHbS) you have severe anaemia with blocked blood capillaries and
tend to have a short life expectancy (rare). Haemoglobin has 146 amino acids. A person
homozygous recessive produces one incorrect amino acid. The haemoglobin is insoluble and the
red blood corpuscles change shape (curved or sickle shape). This causes the breakdown and
clumping of red blood, which leads to paleness, weakness, heart failure, severe pain, damage to
brain and other organs and very often death.
Treatment = blood transfusion. Maybe a bone marrow transplant may provide a permanent cure
in the future.
If heterozygous (HbAHbS) – sickle cell trait – you have generally good health, with only some
anaemia and tiredness. Heterozygotes have the advantage of being resistant to severe forms of
malaria.
 Cystic fibrosis – the inability to remove mucus from the lungs.
The gene codes for a protein found in cell membranes that controls the flow of chloride ions into
and out of the cell. This results in thick mucus that clogs the lungs and stops the correct
functioning of the pancreas and liver.
Chromosome mutation
- a change in chromosome number or alteration of genes within a chromosome.
Potentially more harmful because of the number of genes involved.
 Change in chromosome number
Tend to be harmful in animals and humans but beneficial in plants e.g. Down Syndrome.
This is due to having 47 chromosomes – an extra ‘number 21’ in every body cell. This happens because
one gamete had an extra copy of this chromosome. During meiosis homologous chromosomes failed to
separate and hence, two of the gametes had no number 21 and two had two copies of the chromosome.
Symptoms: ‘Mongoloid’ appearance, large tongue, small hands and fingers, defects in heart, almond-shaped
eyes, round faces, mental retardation, reduced resistance to disease.
Variation
SEC Sample Paper HL
2.
Select the correct term from the following list to match each of the terms in column A and write it in
column B.
protein, enzyme, uracil, sap, ethanol, mutation, thymine, chlorophyll.
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Variation
A
B
DNA
Ribosome
Vacuole
Fermentation
RNA
Active Site
Variation
2004 HL
2.
The diagram shows the distribution of heights in a group of men between the ages of 18 and 23
Distribution of human heights
What term is used by biologists to describe differences within a population with respect to features such
as height? …………………….………...
State two factors that could be responsible for the differences shown.
1.
…………………………………………………………………………………………….
2.
…………………………………………………………………………………………….
Would you expect a similar distribution if the students were weighed instead of being measured for
height?
………………………………………………………………………………………………………………
Explain your answer. ………………………………………………………………………………………
………………………………………………………………………………………………………………
What is a mutation? ………….……………………………………………………………………………
State one cause of mutation. ………………………………………………………………………………
Give an example of a condition, found in the human population, that results from a mutation.
……………………………………………………………………………………..………………………
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Variation
2012 HL
6.
(a)
In genetics, what is meant by the term variation?
_______________________________________________________________________________________
(b)
Variation can result from mutation. Name one other cause of variation.
_______________________________________________________________________________________
(c)
Name two types of mutation.
(i) ____________________________________
(d)
(ii) _____________________________________
Name two agents responsible for increased rates of mutation.
(i) ____________________________________
(e)
(ii) _____________________________________
Briefly explain the significance of mutation in relation to natural selection.
_______________________________________________________________________________________
_______________________________________________________________________________________
SEC Sample Paper OL
3.
Select the correct term from the following list to match each of the terms in column A and write it in column B.
liver, variation, lipid, haploid, sap
A
B
Cell membrane
Vacuole
Mutation
Organ
Gamete
2004 OL
3.
Indicate whether the following are true (T) or false (F) by drawing a circle around T or F.
Chromosomes are made of DNA and lipid
Organisms of the same species can usually produce fertile offspring
T
T
T
F
F
F
RNA is not found in ribosomes
T
F
2007 OL
4.
Complete the following sentences by adding the missing word or symbols or number.
(a)
Genetics is the study of ………………………………………………………………………………
(b)
In a woman the sex chromosomes are XX; in a man they are ………………………………………
(c)
If the diploid number in a cell is 46, the haploid number is …………………………………………
(d)
In order to make proteins, DNA is first transcribed as messenger ………………………………….
(e)
A change in the genetic material of an organism is called a …………….…….…………………….
Answer
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Variation
2010 OL
4.
Choose each term from the following list and place it in Column A to match a description from Column B.
The first one has been completed as an example.
Dominant
Column A
Genetics
Gamete
Gene
Mutation
Genetics
Genotype
Column B
The study of biological inheritance
(i)
The genetic make up of an individual
(ii)
A sex cell
(iii)
A change in the structure of DNA
(iv)
A part of DNA with information to make one protein
(v)
The allele expressed in the heterozygous condition
2006 HL
12.
(a)
(i)
Explain the following terms as used in genetics: species, variation.
(ii) Give one cause of genetic variation.
(9)
2009 HL
(c)
(i)
Explain the term species.
(ii) Within a species a considerable degree of variation is usually seen.
1. What is meant by variation?
2. State two causes of variation.
(iii)
What is the significance of inherited variation in the evolution of species?
(iv)
State two types of evidence used to support the theory of evolution.
Answer
2009 OL
10.
(c)
2012 OL
(i)
(b)
What is meant by the term species?
Read the paragraph below and answer the questions that follow.
The rabbit in the photograph has no pigment in its skin, fur or eyes. This is due to
an inherited condition known as albinism. Such animals are unable to produce
melanin, a protein pigment that gives colour to the skin, eyes, fur or hair. This
condition makes an animal more likely to be preyed upon.
Albinism is caused by genetic mutation. The gene that causes albinism (lack of
pigment) is a recessive gene. If an animal has one gene for albinism and one gene
for pigmentation, it will have enough genetic information to make pigment and
the animal will not have this disorder. However, if both genes are recessive the
result is albinism. At least 300 species of animal have albino individuals e.g.
rabbits, turtles, squirrels, deer and frogs.
(i) What are the main characteristics of albinism?
(ii)
What is meant by the term recessive gene?
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Variation
(iii)
(iv)
What is a mutation?
Mutations can lead to variation in organisms. Variations that make an organism better adapted
to its
environment can lead to evolution.
/1. What is meant by evolution?
2.
Name one of the scientists who first explained how evolution occurs by natural selection.
3. Give one source of evidence for evolution.
(v)
People with albinism should always apply a high-factor sunscreen when going outdoors and must avoid strong
sunshine. Suggest a reason for these precautions.
2004 HL Sample Q2
6(3)+2
A
DNA
Ribosome
B
Thymine
Protein
Vacuole
Fermentation
Sap
Ethanol
RNA
Uracil
Active Site
Enzyme
Variation
Mutation
2004 HL Q2
2.
2(4)+6(2)
Variation
Genetic or examples / environment or examples / age /
[NOTE – environment + food = 1 point]
any two points
Yes or No or implied in text (on this line)
Explanation:
Weight is also determined by genetic or environmental factors [for ‘yes’
above]or valid reason e.g. reference to eating habits or exercise [if ‘no’
given above]
[Note: reason must match the Yes/No above]
Change in genetic makeup (or in DNA, in gene, in chromosome, etc.)
Radiation or chemical or viruses or carcinogens or named example of
any one of these [allow smoking]
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Variation
Down’s syndrome or other correct condition e.g. cancer or stripe in eye
colour [any spontaneous change - one incorrect does not cancel]
2012 HL Q6
6.
2(7) + 6(1)
(a)
Differences (within a population or within a species or between individuals)
(b)
Sexual reproduction or meiosis or independent assortment or environmental
(c)
(i)
Gene (mutation)
(ii)
Chromosome (mutation)
(d)
(i)
(ii)
Example 1
Example 2
(e)
New phenotypes or new types or new features /
Better adapted or survival of the fittest (or advantageous)
or less well adapted (or disadvantageous)
2004 OL Sample Q3
3.
5(4)
Cell membrane
Lipid
Vacuole
Sap
Mutation
Variation
Organ
Liver
Gamete
Haploid
2004 OL Q3
3.
2(5)+5(2)
(a) T
(b) T
(c) F
(d) T
(e) F
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Variation
(f)
F
(g) T
2007 OL Q4
4.
5(4)
(a) inheritance or heredity or chromosomes or genes or DNA
(b) XY
(c) 23
(d) RNA
(e) mutation [allow genetic engineering]
2010 OL Q4
4.
(i)
Genotype
(ii)
Gamete
(iii)
Mutation
(iv)
Gene
(v)
Dominant
2(7) +
3(2)
2006 HL Q12
12.
(a) (i)
(ii)
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species: interbreeding results in fertile offspring
variation: difference between members of species or population
sexual reproduction / meiosis / mutation or agent
3
3
3
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